Operational amplifiers (op-amps) commonly employ differential amplifier (diff-amp) input stages which have a full response to differential signals while rejecting common mode inputs. Common mode rejection ratios of 100 db are typical in commercial integrated circuit (IC) diff-amps.
When a diff-amp has one input grounded and the other input driven, it typically has a very narrow linear range. The input will limit in terms of output swing for inputs of a few tens of millivolts (typically, about .+-.60 mv). Thus, any input signal in excess of limiting will result in rail-to-rail output-current swing and what is called "hard limiting" occurs. In some applications hard limiting is undesirable and it is desirable to have the diff-amp output be an essentially-linear response to the input. This linearity should extend over a substantial range.
The slew rate of an op-amp is defined as the change in output voltage per unit time in response to a large input-step function. With an undegenerated bipolar diff-amp for the input, the slew rate is given by: EQU dV.sub.o /dt=4IIf.sub.o kT/q
where:
k=Boltzman's constant PA1 T=Absolute temperature PA1 q=Electron charge PA1 f.sub.o =op-amp unity gain bandwidth
At 27.degree. C. kT/q is close to 26 millivolts. Therefore, the typical slew rate for an op-amp having a conventional undegenerated bipolar transistor input stage is about 0.32f.sub.o. This means that a 10 MHz bandwidth op-amp will typically display a 3.2 volts per microsecond slew rate. This limitation can be circumvented by using degenerative resistance in the input stage, but this degrades the offset voltage and the noise specification. Accordingly, it is desired to avoid degenerative resistance.